Such remote control is widely used at present. It permits, for example, remotely opening or closing vehicle doors. To do this, and so as to avoid any fraud or unauthorized opening, the codes emitted, between the key and the computer controlling locking/unlocking of the doors, no longer use fixed codes but so-called changing codes.
Thus, if the code emitted by the key is always the same, as was previously the case, it would suffice for an intruder to recopy this code so as thereafter to have access to the vehicle without difficulty. To avoid this fraud, the codes emitted between the key and the computer are so-called changing codes. Such codes are calculated at each emission from an algorithm known to the key and the computer as a function of the preceding code plus an increment. Such changing codes give good results, because the code emitted by the key toward the given vehicle is never the same. Thus, it no longer suffices to copy an emitted code to have access to the vehicle.
However, this type of code has a serious drawback, namely a risk of desynchronization between the key and the computer. Thus, each of these devices (key and computer) computes according to a same algorithm a code, but this in an entirely independent manner. Thus, if for any reason the last code emitted by the key and the last code received by the computer are not the same, it is no longer possible remotely to control the computer from the key. This happens particularly when one presses on the key. There is a desynchronization. Slight desynchronizations (which is to say if the key is fast by one-fiftieth of a code on the computer) have already been solved and this will not be treated in detail here. On the other hand, large desynchronizations (the computer and the key no longer have, in their code lists, common elements) remain a serious problem.
Publications in the prior art disclose processes for resynchronization between the key and the computer in the case of large desynchronizations. There can thus for example be cited FR-2 799 862, which discloses a process for automatic resynchronization in which, when desynchronization has been detected, the computer emits a first message comprising a desynchronization code, a challenge code and its associated response code. The key then emits its own response to the challenge code sent by the computer and memorizes the challenge code as a new base code. The computer receives and analyzes the message sent by the key and verifies that the response code sent by the key is correct and in this case the computer uses the challenge code as the new base code.
U.S. Pat. Nos. 5,191,610 and 5,646,996, and EP0 857 842 disclose other processes permitting resynchronization between an emitter and a receiver. In all these documents, the resynchronization is generally provided to be carried out by a transponder or the like. The computer then knows that the key with which it communicates is desynchronized.
In the present invention, we deal with the case in which the vehicle is provided with a so-called hands free access system. The user of the vehicle is then simply provided with an electronic card, also called a badge, which is detected and recognized by a control and management device associated with the antennas onboard the vehicle. If the card is identified by the control and management device as being a card authorized by the vehicle, the bearer of this card can enter the vehicle simply by operating a door handle. In certain systems, it is also provided that the carrier of the badge can start the motor of the vehicle by simply action on a button.
The problem which thus arises is that there is no longer a transponder to carry out resynchronization. In opening and starting systems which are not hands free systems, there are used changing codes to open and close the vehicle. The resynchronization takes places when the mechanical key is emplaced in the lock to effect starting of the vehicle. With a hands free system, this maneuver is no longer carried out and starting the vehicle is no longer controlled by means of a transponder but by a communication of the challenge/response type between a badge and the management system of the vehicle.